Occasionally I am asked about how to determine coverage with a boom, especially when we see a municipality or DOT over-applying. How do you prevent using too much?
To start, take a look at the spray heads. Your spray heads are most likely marked with 2 numbers. One is a flow rate in gallons or liters per minute. The second is the width of the fan in degrees. You will have to do some math. How far away from the ground do you want to run the boom? Usually 18-24 inches above the ground will give you good spread but limit how much product gets blown away in a light wind during application. That will let you calculate the width of the individual fan pattern based on the degrees width of the fan. (triangle geometry) Then you decide how much overlap you need. Then position your spray heads a proper distance apart to get the coverage with overlap. Most commercially made booms have done this part for you.
Then you have to look at application rate. This is where the adventure begins.
For example, we use a 7 ft boom with 6 evenly spaced spray heads that sprays about 8.5 ft wide. Then you need to look at flow rating. Say a spray head is rated at 1.5 gal per minute at 40psi. (our typical) So your total gallons per minute for the boom is 9. (6 spray heads, 1.5 gallon per minute each)
Then you look at speed. How many feet per minute are you traveling ?. At 10 miles per hour you are traveling (5280 ftx10mph)= feet traveled in 1 hr. Then divide by 60 minutes in an hour and you get 880 ft per minute. Now you take your feet per minute times the width of the spray pattern for the whole boom, (8.5 ft wide spray pattern) and you get the square feet you are covering every minute with that boom at 10 mph. 7480 sq ft. Yes, all this to calculate how many square feet you cover in 1 minute at 10 mph.
Now you divide the sq ft covered by the gallons used in that same minute to get a sq ft per gallon coverage rate. In the example 7480/9= 831 sq ft per gallon at 10 mph
That is a little too thick coverage for most liquids. So, you put in lower flow rate spray heads, or speed the truck up above 10mph, or drop the pressure a little (lower psi) to get nearer 1000 sq ft per gallon which is usually a good starting place. **Check with your liquid manufacturer for application rates according to temp and road surface condition.**
Hoowee what a pain. But worth it to be sure you coverages are right for the boom you have.